It was in 1929 that Salvador Dalí brought his attention to bear on the internal mechanism of paranoiac phenomena and envisaged the possibility of an experimental method based on the sudden power of the systematic associations proper to paranoia; this method afterwards became the delirio-critical synthesis which bears the name paranoiac-critical activity.

Paranoia:

delirium of interpretive association bearing a systematic structure.

Paranoiac-critical activity:

spontaneous method of irrational knowledge based on the interpretive critical association of delirious phenomena.

Along with its predecessor, dada, surrealism sought to challenge the
mechanistic rationalism that dominated the politics and academia of the early 20th Century, seeing in it the genesis of the first world war and other dire social and cultural trends.

Where Dada and surrealist automatism were content to encounter and exhibit the irrational in a spontaneous psychic effort on the part of the artist, producing works which aim at eluding any interpretive schema, the "veristic" strand of surrealism was happier with the use of symbolic elements, and came to be more aligned with the psychoanalytic idea of a "dream language" spoken by the unconscious, whose unplanned irruptions into waking life constitute the phenomena of delirium and madness, but which is nonetheless the true source of the artistic imagination.

Dalí's contribution to developing surrealist theory was the idea that in what he calls the "interpretive delirium" of paranoia, no straightforward hallucination is involved (seeing what is not there), merely an unusual ascription of significance and meaning to what is perceived, as for example when your paranoid friend hears just the same words on the radio that you do, but is able to hear all sorts of "hidden meanings" that you can't (seeing what is there, but differently).

In the essay The Rotten Ass, 1929 (which I think marks the first appearance of the phrase paranoiac-critical in print) Dalí writes:

As far removed as possible from the sensory phenomena that can be thought of as more or less connected to hallucination, paranoid activity always makes use of verifiable, recognizable materials. It is enough for someone in the grip of an interpretive delirium to link the meanings of heterogeneous paintings that happen to hang on the same wall for the real existence of such a link to become undeniable. Paranoia uses the external world to validate an obsessive idea, with the troubling result of validating its reality to others. The reality of the external world serves as illustration and proof of the paranoid idea and is subservient to the reality in our minds.

As can be seen from that quote, Dalí was pleased to grant (at least for purposes of his artistic programme) that the "hidden meanings" have equal status with the conventional interpretations - their "real existence" is "undeniable" - and it's possible to see in his work a desire to exhibit this undeniable presence directly and concretely for the viewer.

The exemplary form of this was for Dalí the "double image" where a single piece of canvas and paint presents equally well two distinct and incompatible images, as in his famous Slave Market with the Disappearing Bust of Voltaire (1940), where the same region of canvas can be seen equally well as either a bust of (18th Century arch-rationalist) Voltaire, or two Dutch merchants (or spanish nuns) walking through an archway - the sky seen through the rounded arch becoming Voltaire's cranial dome, the heads and hats of the merchants (or nuns) doubling as Voltaire's crinkly pop-out eyes, and so on.

This is an extremely developed example, distinguished by its full-on realism, of a kind of optical illusion known as a bistable ambiguous image. Other less illustrious examples are the necker cube and the duck-rabbit. The burden of generating such exquisite ambiguity fell on the paranoiac part of Dalí's method, and it's an intriguing question as to what exactly was involved in this. Dalí speaks vaguely of inducing in himself a paranoiac form of mentation, without giving any practical details, but it might be a mistake to assume that there was nothing more to be said.

This is difficult territory, since access to the irrational was the cornerstone of surrealist credibility and, to some, automatic proof of genius; dada and surrealism had a tendency to hero-worship the insane. Dalí famously said: "The only difference between myself and a madman, is that I am not mad!" and it's certainly true that his extraordinary productivity of "double images" and other "paranoiac" visuals indicates an extraordinary flexibility of perception (let's say) at the very least.

Whether we should understand this as Dalí literally inducing in himself a form of insanity or other altered state, though retaining, as per the critical part of the equation, the ability to produce out of this an artistic work rather than an entrenched delusory belief system, must be weighed against Dalí's undoubted showmanship and self-publicity. Certainly such a literalist reading is encouraged by Dalí, who also remarked that "The only difference between me and the surrealists is that I am a surrealist!", with some justification, if it's taken as a claim about the extent to which he put irrationalism into practice in his daily life.

In any case, Dalí's methodology only makes sense if we understand that the paranoiac-critical artist unleashes in himself in some fashion the kind of unusual perceptions that are found in madness or delirium and then is able to bring these effectively to the viewer through their careful presentation and embedding in the associative framework of some artistic work. Dalí's virtuosic technique comes to the fore in the latter task, either in full-blooded trompe l'oeile realism or in masterful adaptations of several classical styles, allowing the creation of the compelling representations which are necessary for the full psychic assault on the viewer's "rationality".

Dalí makes it reasonably clear in his "Rotting Ass" essay that this rationality, the consensus reality of modernism, is to be regarded itself as a paranoiac delusion of the worst and most entrenched kind, "in which the cruelest, most violent automatism achingly reveals a hatred of reality and a need to seek refuge in an ideal world that is common in childhood neurosis". By exhibiting his carefully framed alternate realities, and nodding towards the contingency and superfluity of perception itself, Dalí hoped to weaken the grip of the collective madness on us all, to "systematize confusion and contribute to the total discrediting of the world of reality" - on Dalí's view, a moral act "in the service of the Revolution."

Dalí, Salvador, The Conquest of the Irrational, 1936. Reprinted in Salvador Dalí: A Panorama of His Art, edited by A. Reynolds Morse. Salvador Dalí
Museum, Cleveland, Ohio, 1974, as cited by Aaron Ross, in The Art of Salvador Dalí: From the Grotesque to the Sublime at:
http://www.dr-yo.com/grot.html